A. Field of the Invention
This invention relates to capacitors and in particular to an electrolytic capacitor counterelectrode having a tantalum surface.
B. Background Art
The counterelectrode in a wet-type tantalum capacitor must present a very low impedance to an AC signal if the capacitance of the anode element is to be the determining factor in the capacitance of the finished capacitor. This is generally accomplished by either the use of a porous tantalum sintered body with high specific capacitance, as in the all tantalum types or by deposition of a spongy layer of a member of the platinum group metals, as in the silver case types.
The ability to produce an effective surface on the case wall of capacitors where the inner surface of the case consists of tantalum is desirable economically in parts using wholly tantalum cases. It is a necessity for laminated case parts where the nature of the laminated structure makes sinter or welding of a porous cathode body into the case impossible.
The ability of tantalum to form a thin, adherent, dielectric oxide is the basis for tantalum capacitors but is also the primary problem in using a tantalum surface for a counterelectrode. It is impossible, from a practical standpoint, to keep a tantalum surface oxide free and the specific capacitance of an oxidized surface, either smooth or etched, is far too low to make a satisfactory counterelectrode.
Furthermore, the presence of even the thinnest of air oxides precludes the use of electrolytic or electroless deposition of a noble metal as has traditionally been done in silver case parts. This is because the air oxide does not conduct electricity and deposition will only take place upon small breaks in the oxide.
A solution to this problem has been to find a method of treating the tantalum surface that destroys its ability to form an oxide. Any method, to be considered, must be physically and chemically compatible with all the materials present in the case construction.
U.S. Pat. No. 4,523,255 issued Jan. 11, 1985 to Rogers teaches destroying the oxide forming ability by converting a thin layer of the tantalum surface to tantalum carbide. This layer did not have any great effective capacitance by itself but, more importantly, its nonmetallic nature limited the choices available for the required secondary treatment. Rogers further taught the use of a carbon layer deposition from a colloidal dispersion of graphite. However, the graphite layer did not adhere well to the tantalum carbide surface and in many ratings relied upon the addition of "depolarizing" agents to the capacitor electrolyte.
U.S. Pat. No. 3,628,103 issued to Booe, disclosed forming on virtually any metal an inner layer of platinum on a cathode of a wet electrolyte capacitor. Tantalum in particular is not specified.